Subsurface imaging radar

Abstract

A method and system for obtaining SAR images with reduced or eliminated surface clutter to detect subsurface targets, the method comprising the following steps:—selecting a first frequency and an incidence angle for the radar signal such that the ratio of surface backscattering to subsurface target backscattering is significantly larger for vertical polarization than for horizontal—obtaining vertically and horizontally polarized SAR images based on the same SAR path exploiting the selected first frequency and viewing angle—weighting and differencing the vertically and horizontally polarized SAR images so that the surface backscattering completely cancels between the two images and only the combination of the target backscattering components remains.

The invention claimed is: 
     
       1. A method of removing surface clutter in SAR radar imaging of subsurface targets, said method comprising the steps of:
 selecting a first frequency and an incidence angle (χ 0 ) for a radar signal such that the ratio of surface backscattering to subsurface target backscattering is significantly larger for vertical polarization than for horizontal polarization; 
 simultaneously obtaining vertically and horizontally polarized SAR images, the obtained SAR images being coincident except for differences in the polarization of each, the obtained SAR images exploiting the selected first frequency and incidence angle for a vertically polarized and a horizontally polarized radar signal; and 
 weighting and differencing, via a control unit, the vertically and horizontally polarized SAR images so that the surface backscattering completely cancels between the two images and only the combination of the target backscattering components remains. 
 
     
     
       2. The method according to  claim 1 , wherein the first frequency of the radar signal and the incidence angle are chosen such that the wavelength of the vertically polarized radar signal is at least one of greater than or equal to the surface roughness. 
     
     
       3. The method according to  claim 1 , wherein the incidence angle prevents shadows arising in the radar signal. 
     
     
       4. The method according to  claim 1 , wherein the incidence angle is chosen to be larger than zero and less than the Brewster angle, wherein an incidence angle of zero is horizontal incidence. 
     
     
       5. The method according to  claim 1 , wherein the horizontal and vertically polarized radar signals are generated by a horizontal and a vertical antenna that conduct registrations in a so called ping-pong mode. 
     
     
       6. The method according to  claim 1 , wherein the first frequency is between 25 and 500 MHz. 
     
     
       7. The method according to  claim 6 , wherein the first frequency is between 130 and 360 MHz. 
     
     
       8. The method according to  claim 1 , wherein transmitting and receiving components are provided and configured to work within a range of 25 m to 5000 m. 
     
     
       9. The method according to  claim 1 , wherein transmitting and receiving components are provided and configured to work within a range of 100 m to 500 m. 
     
     
       10. The method according to  claim 1 , wherein an adaptive minimum energy method is used to weight and difference the vertically and horizontally polarized SAR images so that the surface backscattering cancels between the two images and only the combination of the target backscattering components remains. 
     
     
       11. The method according to  claim 10 , wherein the method comprises the following steps:
 with the aid of an aircraft in flight ( 205 ):
 obtaining ( 210 ) a HH complex SAR image F H (x, y); 
 
 obtaining ( 215 ) a VV complex SAR image F V (x, y);
 selecting ( 305 ,  350 ), within the images, an area T that appears to be homogenous; 
 calculating ( 310 ) a γ min  such that an energy function (E) is minimized, wherein: 
 
 
       
         
           
             
               
                 E 
                 = 
                 
                   
                     
                       ∫ 
                       ∫ 
                     
                     T 
                   
                   - 
                   
                     
                       
                          
                         
                           
                             
                               F 
                               H 
                             
                             ⁡ 
                             
                               ( 
                               
                                 x 
                                 , 
                                 y 
                               
                               ) 
                             
                           
                           - 
                           
                             
                               γ 
                               min 
                             
                             ⁢ 
                             
                               
                                 F 
                                 V 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                           
                         
                          
                       
                       2 
                     
                     ⁢ 
                     dxdy 
                   
                 
               
               ; 
             
           
         
          and
 forming ( 320 ) a ground clutter suppressed SAR image ΔF(x, y) by forming the expression:
   Δ F ( x,y )= F   H ( x,y )−γ min   F   V ( x,y ).
 
 
 
       
     
     
       12. The method according to  claim 11 , wherein subsurface targets subsequently are detected ( 325 ) by applying one of CFAR thresholding, ICD or CCD methods on ΔF. 
     
     
       13. A SAR system for providing SAR images having removed surface clutter to improve detection of subsurface targets, said system comprising:
 a selection unit configured for selecting a first frequency and an incidence angle (χ 0 ) for a radar signal such that the ratio of surface backscattering to subsurface target backscattering is significantly larger for vertical polarization than for horizontal polarization; 
 a linear combiner configured, via a linear combiner control unit, for:
 simultaneously obtaining vertically and horizontally polarized SAR images, the obtained SAR images being coincident except for differences in the polarization of each, the obtained SAR images exploiting the selected first frequency and incidence angle for a vertically polarized and a horizontally polarized radar signal; and 
 weighting and differencing the vertically and horizontally polarized SAR images so that the surface backscattering completely cancels between the two images and only the combination of the target backscattering components remains. 
 
 
     
     
       14. The system according to  claim 13 , wherein the selection unit is further configured for selecting a homogenous test area T as input to a gamma-min finding unit. 
     
     
       15. The system according to  claim 13 , further comprising a target detector and a display unit for detecting and visualizing detected targets to an operator. 
     
     
       16. The system according to  claim 13 , further comprising a ping-pong control unit connected to the transmitters to make the transmitters send in ping-pong mode.